The present invention provides a method for manufacture of noble metal alloy catalysts. The method is based on the use of polyol solvents as reaction medium. The catalyst manufactured according to the invention are characterized by a high degree of alloying and a very small crystallite size. They can be used as electrocatalysts for polymer electrolyte membrane fuel cells (PEMFC), direct-methanol fuel cells (DMFC) or similar electrochemical devices. Furthermore, they are suitable as catalysts for CO-oxidation reactions (PROX) or exhaust gas purification.
In principle, fuel cells are gas-operated batteries, in which the chemical energy obtained from the reaction of hydrogen and oxygen is converted directly into electrical energy. The present invention describes catalysts for PEM fuel cells (PEM=polymer electrolyte membrane), which are suitable for operation with hydrogen-containing gases or with methanol (DMFC=direct methanol fuel cell). Fuel cells are gaining increased importance as mobile, stationary or portable power sources.
Electrocatalysts based on platinum (Pt) are routinely used on the anode and cathode side of PEM fuel cells. They comprise of finely divided noble metal particles, which are deposited on a conductive support material (generally carbon black or graphite). Normally, the concentration of noble metal is in the range from 10 to 90 wt. %, based on the total weight of the catalyst.
Conventional platinum anode catalysts are very sensitive to poisoning by carbon monoxide (CO). Therefore the concentration of carbon monoxide (CO) in the anode gas has to be reduced to the ppm-range in order to prevent performance losses in the fuel cells due to poisoning of the anode catalyst. It is known that the tolerance of a platinum catalyst to poisoning by carbon monoxide (CO) can be improved by alloying the platinum with ruthenium (Ru). Generally, this means that oxidation of the carbon monoxide adsorbed on the platinum to carbon dioxide (CO2) takes place and the CO2 is then readily desorbed.
For the cathode side of PEMFCs, frequently pure Pt/C catalysts are used. As some Pt alloy catalysts, comprising Pt and the base metals Co, Cr or Ni offer an activity enhancement by the factors of 1.5 to 3 (ref to U. A. Paulus, A. Wokaun et. al, J. Phys. Chem. B, 2002, 106, 4181-4191), these alloy compositions are more frequently used for cathode catalysts.
The present invention refers to the manufacture of nano-sized noble metal catalysts via the colloid route. Various processes for manufacture of noble metal colloids in water-based systems are known:
EP 1 175 948 discloses noble-metal containing nanoparticles, which are embedded in an aqueous solution of a temporary stabilizer. The nanoparticles are manufactured by reduction of chloride-free precursor compounds in water in the presence of a polysaccharide. The nanoparticle materials are used for catalyst inks, catalyzed ionomer membranes and for the manufacture of electrocatalysts.
U.S. Pat. No. 5,925,463 describes metal, bi-metal and multi-metal colloids with particle sizes up to 30 nm. The colloids are stabilized by water-soluble stabilizers.
EP 423 627 B1 is directed towards the preparation of microcrystalline or amorphous metal or alloy colloids obtainable by reduction with metal hydrides in organic solvents.
EP 796 147 B1 and DE 44 43 705 disclose surfactant-stabilized colloids of mono- and bimetals of the groups VIII and IB of the Periodic System of the Elements (PSE) having particle sizes in the range of 1 to 10 nm. They are prepared by reduction in water in the presence of strongly hydrophilic surfactants.
EP 924 784 B1 teaches alloyed PtRu/C catalysts with a medium particle size in the range of 0.5 to 2 nm. These alloy catalysts were prepared by fixing a PtRu colloid (obtained according to EP 423 627 B1) on a carbon black support at temperatures up to 50° C. The degree of alloying is relatively low, a lattice constant of 0.388 nm is reported.
More specifically, the present invention refers to a catalyst manufacturing process in polyol solvents as reaction medium (the “polyol process”). Similar processes have been recently published in the patent literature.
JP 11 246 901 provides a method for producing stable metal particles deposited on a porous carrier by dissolving a metal salt in a polyhydric alcohol (“polyol”). A pH-adjusting agent is added to adjust the pH value to prevent agglomeration of the particles.
U.S. Pat. No. 6,551,960 discloses the preparation of supported, nanosized catalyst particles, particularly PtRu particles, via a polyol process. A solution of platinum and ruthenium compounds in a polyhydroxylic alcohol solvent is mixed with an electrically conductive support material and subsequently heated to a temperature in the range of 20 to 300° C. to reduce the Pt and Ru to a zero valence state and to form PtRu catalyst particles with less than 1 micron particle size. The present inventors have reproduced this method. It was found that, due to the different reaction rates for the two elements in the process employed, the PtRu alloy formation is not complete (i.e. a low degree of alloying is obtained).
US 2003/0104936 A1 describes nanoparticle catalysts and a method of making them in a polyol process. A solution of metal chlorides of one or more catalyst metals in a polyalcohol solvent is converted to a colloidal particle suspension by raising the pH and heating the solution. This colloidal suspension is then combined with the support material and the nanoparticles are deposited by lowering the pH of the suspension. A mixture of metal compounds is used and the metal compounds are simultaneously reduced.
U.S. Pat. No. 5,856,260 refers to the preparation of high activity catalysts. A solution of polyol is employed to impregnate and disperse a compound of a catalytic metal on an inorganic oxide support. Noble metal containing alloys are not disclosed.